1. Field of the Invention
The present invention relates to an apparatus and a method for measuring concentrations of light-absorbing substances in an aqueous solution. More specifically, the invention relates to an apparatus and a method for optically measuring concentrations of components, which are applicable to measurement of, for example, concentrations of glucose and hemoglobin in blood, and concentrations of protein, bilirubin and saccharide in urine as well as concentrations of components in beverages.
2. Description of the Prior Art
It is well known that the relation between concentration of a solution and light absorption can be expressed by the following Lambert-Beer's law: EQU A.sub.c =Log.sub.e (I.sub.o /I.sub.t)=.SIGMA..alpha..sub.i C.sub.i L(1)
where
A.sub.c =absorbance, PA1 I.sub.o =quantity of incident light, PA1 I.sub.t =quantity of transmitted light, PA1 .alpha..sub.i =extinction coefficient of i component in the sample, PA1 C.sub.i =concentration of i component in the sample, and PA1 L=optical path length within the cell containing the sample. PA1 (i) a step for measuring quantity of transmitted light of a wavelength .lambda. that has been emitted from a light source and transmitted through a cell which does not contain a sample and calculating a quantity of incident light I.sub.o by the following equation: EQU I.sub.t =I.sub.o t.gamma. PA1 (ii) a step for calculating a transmissivity t.sub.s that depends on the refractive index of a reference concentration sample to the light of wavelength .lambda., PA1 (iii) a step for measuring a quantity of transmitted light I.sub.to of the light of wavelength .lambda. when an optical path length L within the cell is set to a reference optical path length L.sub.o, in a state that the cell contains the reference concentration sample, PA1 (iv) a step for calculating a value .gamma..sub.o of .gamma. for light with the reference optical path length L.sub.o and the wavelength .lambda. by the following equation: EQU I.sub.t =I.sub.o t.gamma. PA1 (v) a step for calculating an optical path length ratio k.sub.p corresponding to an optical path length L.sub.p at which quantity-of-light measuring sensitivity S reaches a maximum with the wavelength .lambda., by the following equation: EQU k=-1/log.sub.e (.gamma..sub.o .delta.) PA1 the quantity-of-light measuring sensitivity S being defined by the following equation: EQU S=dI.sub.t /d C.sub.i PA1 (vi) a step for calculating an optimum optical path length L.sub.p.lambda. at which the quantity-of-light measuring sensitivity S reaches a maximum with the wavelength .lambda., by the following equation: EQU k=L/L.sub.o PA1 the above steps being performed repeatedly a plurality of times with the wavelength .lambda. varied, to calculate values of quantity of transmitted light at positions of optimum optical path lengths L.sub.p.lambda. corresponding to the varied wavelengths .lambda., wherein concentration of a component contained in the sample is calculated by multivariate analysis based on the calculated values of quantity of transmitted light. PA1 (i) a step for measuring quantity of transmitted light of a wavelength .lambda. that has been emitted from a light source and transmitted through the cell which does not contain a sample and calculating a quantity of incident light I.sub.o by the following equation: EQU I.sub.t =I.sub.o t.gamma. PA1 (ii) a step for calculating a transmissivity t.sub.s that depends on the refractive index of the light of wavelength .lambda. in a reference concentration sample, PA1 (iii) a step for measuring a quantity of transmitted light I.sub.tL of wavelength with an optical path length L within the cell containing the reference concentration sample, PA1 (iv) a step for calculating absorbances A.sub.c.lambda. to the light of wavelength .lambda. at portions of the cell corresponding to different optical path lengths L, by the following equation: EQU log.sub.e (I.sub.o /I.sub.t)-log.sub.e (1/t)-.alpha..sub.c l=A.sub.c PA1 (v) a step for determining an absorbance A.sub.c.lambda.p equal to log.sub.e e from among the absorbances A.sub.c.lambda. to the light of wavelength .lambda., and PA1 (vi) a step for storing an optical path length L.sub.p corresponding to the absorbance A.sub.c.lambda.p to the light of wavelength .lambda., the above steps being performed repeatedly a plurality of times with the wavelength .lambda. varied, to calculate values of quantity of transmitted light at positions of optimum optical path lengths L.sub.p.lambda. corresponding to the varied wavelengths .lambda., wherein concentration of a component contained in the sample is calculated by multivariate analysis based on the calculated values of quantity of transmitted light.
With regard to this equation, the concentration can be determined by performing multivariate analysis based on measured values of quantity of transmitted light on multiple wavelengths.
Since the concentrations of each component is determined based on measured values obtained by a measuring system, the accuracy of the concentration is limited by the measurement accuracy of the measuring system. Therefore, to determine the concentrations of components, particularly accurate concentrations of trace amounts of components out of the components to be measured, the measuring system is required to have an enhanced accuracy of measurement. Thus, the accuracy of the concentrations determined has a close relation to the S/N ratio of the measuring system.
Japanese Patent Laid-Open Publication No. 63-144237 (1988) has disclosed a method for measuring absorbances and an apparatus for the same, which are applicable to quantitative determination of trace amounts of organic and inorganic components in a sample solution.
This apparatus comprises a cell for a spectrophotometer having different cell lengths (i.e., optical path lengths, which herein mean distances over which light travels within a cell), a feeder for feeding a sample solution to the cell, a switching unit for switching over the optical path length, and a controller for controlling the feeder and the switching unit. According to this apparatus, measurement of absorbances of sample solutions is carried out based on the quantity of transmitted light at a portion of the cell corresponding to the short optical path length for high concentration samples or the quantity of transmitted light at another portion of the cell corresponding to the long optical path length for low concentration samples.
The above measuring method and apparatus incorporate a cell for a spectrophotometer, actually a triangular cell, having different optical path lengths, wherein the optical path length is switched over so that the absorbance falls within a range suited for measurement. Thus, the measuring method and apparatus are intended to accomplish a wide range of concentration measurement from low concentration sample solutions to high concentration sample solutions without necessitating dilution of the sample solution or exchange of the cell.
In the above measuring method and apparatus, the optical path length is switched to a path length such that the absorbance falls within a range suited for measurement depending on differences in concentration among sample solutions, without changing the wavelength of light applied to samples. However, for example, when one component contained in the samples is of almost the same concentration among the samples, like glucose in blood, the method and apparatus have a disadvantage that they could not enhance the measurement accuracy of the concentration of this component.